1. Field of the Invention
The present invention relates to a method of preventing silicide spiking, and more particularly, to a method of preventing the occurrence of silicide spiking to a gate of an embedded dynamic random access memory (EDRAM) cell.
2. Description of the prior Art
Dynamic random access memory (DRAM) is composed of a memory array region and a logic circuit region. With increasing integration of electrical circuit elements, the trend of manufacturing semiconductor integrated circuits is to integrate memory array region and high-speed logic circuit elements into a single chip to form an embedded memory. The embedded memory not only significantly reduces the circuit area but also greatly increases the signal processing speed.
In order to improve the electrical performance of the embedded memory, a silicide layer is often required in a gate structure to reduce contact resistance of the gate. However, during the formation of the gate, silicide spiking occurs. The silicide spiking is resulted from the solubility of metal such as titanium (Ti) or aluminum (Al) in silicon. Specifically, metal ions in the silicide layer frequently penetrate through the interface of the silicide layer and the polysilicon layer to diffuse into the polysilicon layer, forming silicide spiking to affect the electrical performance of the gate. Currently, a method to prevent silicide spiking is to form a barrier layer on the polysilicon layer prior to the deposition of the silicide layer. Thus, by using the barrier layer, metal ions are prevented from diffusing downward to induce silicide spiking.
Please refer to FIG. 1 and FIG. 2 of schematic diagrams of fabricating a wire according to the prior art. As shown in FIG. 1, a silicon oxide layer 12, a polysilicon layer 14, a barrier layer 16 and a silicide layer 18 are formed, respectively, on a surface of a semiconductor substrate 10. Normally, the barrier layer 16 is a silicon nitride layer and the silicide layer 18 is a titanium silicide (TiSi2) layer. A conventional method to form the barrier layer 16 is to use a physical vapor deposition (pVD) process, such as reactive sputtering method, with argon gas and nitrogen gas as reactive gases to bombard a metal target to release titanium atoms. The released titanium atoms thereafter react with nitrogen atoms that are ionized by the plasma, forming titanium nitride on the surface of the polysilicon layer 14. However, the structure of the barrier layer 16 formed by reactive sputtering method is seldom tight enough to prevent metal ions in the silicide layer 18 from diffusing downward in a subsequent thermal process. Thus, a spike 19 of the silicide layer 18 is produced to intrude into the barrier layer 16, and even into the polysilicon layer 14.
Subsequently, as shown in FIG. 2, a photolithographic and etching process is performed. A photoresist layer (not shown) is coated on the surface of the semiconductor substrate 10 to define gate patterns in the photoresist layer. According to the patterns in the photoresist layer, portions of the silicide layer 18, portions of the barrier layer 16, portions of the polysilicon layer 14 and portions of the silicon oxide layer 12 are removed down to the surface of the silicon substrate 10, thus forming a gate 20. Finally, fabricating processes to form other electrical elements, such as source and drain, are performed to complete the MOS transistor of the EDRAM cell. The details of these processes are not the major concern and are thus omitted for simplicity of description.
As the physical vapor deposition of the prior art cannot provide a tight barrier layer 16, the spike 19 occurs on the interface between the silicide layer 18 and the barrier layer 16 and induces leakage currents to affect the electrical performance. In addition, portions of the spike 19 penetrating through the barrier layer 16 also destroy stress properties of the whole barrier layer 16, thus reducing the reliability of the gate 20.
It is therefore an objective of the present invention to prevent the occurrence of a spike in a silicide layer as in the prior art.
It is another objective of the present invention to provide a method of improving a ring oscillator period of an EDRAM cell and increasing the reliability thereof.
According to the claimed invention, a polysilicon layer is formed on a semiconductor substrate followed by performing a collimator physical vapor deposition process to form a barrier layer on the polysilicon layer. A rapid thermal process (RTP) is then performed to tighten the structure of the barrier layer. Finally, a silicide layer is formed on the barrier layer.
It is an advantage of the present invention that the collimator is used as an assistant to form the barrier layer and the RTP process is performed to tighten the barrier layer""s structure after the deposition of the barrier later. As a result, the silicide layer is prevented from occurring spiking, the contact resistance of the polysilicon layer is reduced, and the production reliability is improved. In addition, as the method of forming the barrier layer of the present invention is employed in an EDRAM cell, especially in a gate of a MOS transistor in the logic circuit region, an obvious decrease in a ring oscillator period of the logic circuit region is made as a result of decreasing the contact resistance of the gate. To be specific, the delay time of the logic circuit region is reduced as a result of decreasing the contact resistance of the gate, thus both the logical computation ability and the electrical performance are greatly improved.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.
FIG. 1 and FIG. 2 are schematic diagrams of fabricating a wire according to the prior art.
FIG. 3 and FIG. 4 are schematic diagrams of fabricating a wire according to the present invention.